New Constraints on Dark Photon Dark Matter with a Millimeter-Wave Dielectric Haloscope

Abstract: Dark matter remains one of the most profound and unresolved mysteries in modern physics. To unravel its nature, numerous haloscope experiments have been implemented across various mass range. However, very few haloscope experiments conducted within millimeter-wave frequency range, which is in the favored mass region for well-motivated dark matter candidates. Here we designed and constructed a millimeter-wave dielectric haloscope featuring a dark matter detector composed of dielectric disks and a mirror. Using this setup, we conducted a search for dark photon dark matter and found no evidence for its existence. Our results established new constraints on the kinetic mixing parameter in the mass range from $387.72$ to $391.03$ $\mu eV$, improving the existing limits by two orders of magnitude. With future enhancements, our system has the potential to explore new parameter space for dark photon as well as axion dark matter within millimeter-wave frequency range.